Lighting modules, such as those for street lighting, using solid state light radiation sources (“Solid State Lighting”, or SSL) can be considered competitive in that they simultaneously meet various requirements in terms of robustness relating to the context of their use “in the field”, namely:                resistance to electrical overstress (EOS),        resistance to thermal dissipation,        long service life, and        mechanical strength.        
The first aspect mentioned above is related to the phenomena of electric overload: proper electrical insulation is important not only for avoiding the harm caused by electrostatic discharge (ESD) events during the assembly of the lighting module or of the corresponding device, but also in relation to electrical overload events such as those caused by lightning.
The second aspect is related to the thermal dissipation properties of the housing which encloses the module, and may require a considerable part of the lighting device to be made of a metal material (such as aluminum) so that it has a certain degree of weight. If the module has low thermal resistance between the connection points of the light radiation sources (such as LEDs) and the thermal dissipation surface of the module, the corresponding device may also have a rather high thermal resistance between the surface in contact with the module and the external environment.
The third aspect relates to the faults that may arise in the module even without any causation by a specific external event. These events may have a negative effect on the service life, either in the form of “soft” faults (the light flux falls below a certain threshold level, without total loss of light emission), or in the form of “hard” faults (the radiation source ceases to emit radiation and acts as either an open or a closed contact).
The fourth aspect relates to the mechanical strength in the conditions of use in the field, and requires the module to meet certain requirements in terms of mechanical performance, in exterior applications for example (resistance to vibration, impact, and the like).
In various designs of lighting devices, of the solid state type for example, the four aspects mentioned above tend to create opposing constraints.
For example, electrical insulation may be achieved by using mechanically robust substrates, with the risk of adversely affecting the thermal dissipation characteristics and increasing the possibility of hard faults; on the other hand, materials capable of providing electrical insulation together with good thermal dissipation characteristics while also reducing the risks of hard faults may be mechanically fragile.
For example, it is possible to use substrates of the PCB (printed circuit board) type, in other words those resembling printed circuits, with metal cores, using high luminosity LEDs as the light radiation sources. Solutions of this kind have good characteristics in terms of thermal dissipation, electrical insulation and mechanical robustness. However, they may have critical aspects due to the differences in the coefficient of thermal expansion (CTE) that may be encountered, for example, between the ceramic packages of high luminosity LEDs and materials such as aluminum (15-20 ppm/° C.). In all cases, there is a risk of increasing the possibility of hard faults in the soldering points between the package and the LEDs and the PCB if the module is subjected to thermal cycles such as those which may occur to a pronounced degree in exterior applications, and the consequent possibility of observing a marked reduction in the service life of the LEDs: for example, as regards the light emission performance (LED lumen maintenance), the specified values of 100 kilohours may fall to values of 20-30 kilohours when measured in the field.
It has been proposed that these problems should be tackled by replacing aluminum with copper, for which the mismatch with the ceramic materials in terms of CTE is lower, at about 10-15 ppm/° C. However, this solution has the drawback of practically doubling the cost compared with solutions in which aluminum is used for making the PCB, which is unacceptable in applications where the cost of the PCB accounts for a significant part of the overall cost of the device.
A performance substantially comparable to that of copper, in terms of the mismatch of the coefficient of thermal expansion (CTE) with respect to packages of ceramic material, can be achieved by using the material known as FR4, although the latter has a low level of thermal dissipation; attempts may be made to counteract this characteristic by providing thermal bridges (“vias”) through the PCB, but this has negative effects on the electrical insulation characteristics.
It has also been proposed that PCB substrates of ceramic material should be used, as these can provide high performance in terms of thermal characteristics, electrical insulation and the service life of the module, but this would have adverse effects on the mechanical characteristics, particularly where the possibility of using large PCBs is being considered.
The use of what are known as “Chip On Board” (CoB) products appears more promising, although these products are uncompetitive, at the present time at least, in terms of the lighting density (known as the cost per lumen), while they do not allow a high chip density in the CoB.
It is also possible to consider the use of medium- to low-power LED sources as light radiation sources, thus enabling non-ceramic packages to be used and increasing the reliability of the soldered connections. However, this solution also has the drawback of a high cost per lumen and rather low resistance to possible corrosion by environmental factors (such as sulfuric components).